OLD RED SANDSTONE OF WESTERN EUROPE. 393 
yards in extent, are profusely covered with fine rippled lines as sharply pre- 
served as if only to-day imprinted on the soft sediment. In many places every 
successive stratum or leaf of rock is thus marked, so that several distinct rippled 
surfaces may be counted in the thickness of a few inches of rock. It is likewise 
observable that the rippling is generally close-set, sometimes not exceeding an 
inch in breadth from crest to crest of the ridges. 
~ More abundant and admirable illustrations of sun-cracks could hardly be 
found than occur along this coast. Broad gently-inclined sheets of rock 
again and again present themselves to view so covered with reticulations as to 
look like tesselated pavements. It may be noticed that the cracks not infre- 
quently descend through many of the fine lamine of deposit for a depth of five 
or six inches with occasionally a breadth of three or four inches. The material 
filling up the interstices abounds with small, occasionally curved pieces of shale. 
These may, no doubt, be regarded as portions of the upper muddy layer which 
cracked off and curled up during desiccation, as may often be observed on 
dried-up pools at the present time. Some pittings, occasionally seen on the 
sun-cracked surfaces, may perhaps represent rain-drops. Altogether, no 
evidence could more conclusively indicate a long-continued, tranquil deposit 
of fie sediment in shallow water, which frequently retired and left wide tracts 
_of muddy shore to be dried and cracked. by exposure to the sun. 
Reserving the consideration of the paleontology of the upper flagstone 
group for a subsequent section, it may be stated here that organic remains 
abound in the strata exposed on the shore between Dunnet Bay and Reay. 
Fragments of fish and coprolites are scattered abundantly through most of the 
flagstones. Some of the calcareous shales are full of Estheria, while traces of 
plants occur in great numbers, though generally in a somewhat macerated 
condition. 
In tracing the succession of beds westwards beyond Thurso, we encounter a 
thick series of strata in which the characters now described are still fully 
developed. They rise, however at Holburn Head into noble vertical walls 
from 100 to 250 feet in height, against the base of which the rapid tides of the 
Pentland Firth are ever chafing. In these precipices the thinly bedded nature 
of the flagstones, their clean-cut joints, and the tardy but not ineffectual 
influence of the weather upon the edges of. the gently sloping strata, can be 
traced on headland and “goe” for many miles. The dip continues very 
steadily towards N.N.W. at angles ranging from 3° to 15° or even 20°, but pro- 
bably averaging not more than 8° or 10°. A marked but strictly local cur- 
vature occurs at the Port of Brims. With this exception, there is hardly any 
interruption of the prevalent dip even so far as the extreme western end of the 
flagstone section. Though the trend of the coast nearly coincides with the 
strike of the beds, nevertheless, as already stated (p. 378), a sufficient divergence 
VOL. XXVIII, PART IL. 5K 

